Welsh Water is to pay a proposed £44.7m after the industry regulator found “serious and unacceptable” breaches in the supplier’s sewage and network services.

Ofwat said Dŵr Cymru Welsh Water failed to properly operate, maintain and upgrade its wastewater network to ensure it could cope with levels of sewage and wastewater, and did not have adequate processes in place or oversight by senior bosses.

Ofwat said the planned enforcement package will include £40.6m to reduce spills at specific overflows and reduce the environmental damage caused, tackle groundwater entering the sewer network, as well as an extra £4.1m to improve river quality in “extremely sensitive catchments”.

Lynn Parker, senior director for enforcement at Ofwat, said: “Our investigation has found serious and unacceptable breaches in how Dŵr Cymru Welsh Water has operated and maintained its sewage works and networks, which has resulted in excessive spills from storm overflows to the environment.

“We now expect them to focus on putting things right so that customers can regain trust in their water company and the critical service they provide. We understand that the public wants to see transformative change.”

Ofwat stressed the package was more than would otherwise have been paid if Ofwat was to fine the firm the £40m it could have done.

It will now consult on the proposed enforcement package, with a deadline for responses of 2 April.

Details of the planned action come as the bills of Welsh Water customers are due to increase again next month, with the supplier having announced rises of 42% by 2029-30. The company serves about 3 million people across Wales and Herefordshire.

A Welsh Water spokesperson said: “We accept the findings of Ofwat’s investigation and apologise for where we have fallen short of the standards that our customers and regulators rightly expect from us.

“We have started a major transformation programme across the company, including within our wastewater services, focused on improving performance, strengthening operational oversight and accelerating investment to deliver better outcomes for rivers and coastal waters.

“The investigation has considered both historic and more recent compliance, and we accept that improvements are needed.”

It marks the latest enforcement action by Ofwat after it said earlier this month it planned to fine South East Water £22m for ‘repeated supply failures’ between 2020 and 2023, affecting more than 286,000 people.

Ofwat said this was the seventh case in its sector-wide sewage investigation, with this proposed enforcement action bringing the total so far amid the probe to more than £300m.